Phosphors for color display systems



Dec. 8, 1970 Em l-ITAL I PHOSPHORS} FOR COLOR DISPLAY SYSTEMS 4' Sheets-Sheet 1 Filed May 19, 1967 FIG. 2.

IIIII.

Dec. 8, 1970 ETAL v 1 3,546,003

PHOSPHORS FOR COLOR DISPLAY SYSTEMS FiledMay 19, 1967 4 Sheets-Sheet 4 United States Patent Office 3,546,003 Patented Dec. 8, 1970 U.S. Cl. 117100 3 Claims ABSTRACT OF THE DISCLOSURE A thin layer of zinc sulfide-containing phosphor particles is positioned in a quartz container supported on a slab of silicon carbide coated graphite material in a reaction chamber in an atmosphere of oxygen and heat is periodically supplied to cyclically vary the temperature of the particles from about 350 C. to 600 C. The barrier layer of zinc oxide thereby formed on the surface of the particles, or a surface layer of zinc sulfide subsequently formed in situ, increases the electron energization threshold of the particles. These particles are utilized together with other phosphor particles with a lower energization threshold in an energy modulated color display system.

BACKGROUND OF THE INVENTION This invention relates to phosphors for color display systems, and more particularly to such phosphors which have increased energization thresholds and to methods and apparatus for making such phosphors.

In recently developed color display systems, electron viewing screens are employed which include phosphor particles of different color light-emitting characteristics and which are respectively differently responsive to electrons of 'ditfering energies or velocities. In such systems, the viewing screen includes a first phosphor (e.g., one which emits light of relatively long wavelengths such as red) which is energized to emit light when struck by electrons having at least a first predetermined velocity or beam energy level, for example, accelerated by a kinescope accelerating voltage of perhaps kv., this being the operating voltage for the red phosphor, although the phosphor turns on or begins to emit light at much lower voltages. The viewing screen also includes particles of a second phosphor, e.g., one which emits a substantial level of a second color light of shorter wavelengths, and preferably complementary in color to that of the first phosphor (such as a cyan colored light), when energized by electrons having at least a second and higher predetermined velocity, e.g., 1S kv., this being the operating voltage for the second phosphor. That is, while the second phosphor begins to emit light at a lower voltage, perhaps at 10 kv., a substantially higher voltage is used to achieve the required light level. If a beam of electrons of the lower velocity, 10 kv., is current modulated in accordance with the red record represented by the red color information signal derived in the receiver of any conventional color television receiver (such as those operating in accordance with the NTSC, SECAM, or PAL systems), a red color image corresponding to the red records is presented on the viewing screen of the kinescope. At electron velocities of 10 kv., the second or cyan light-emitting phosphor will not be significantly energized to emit light, although it may be just turning on. By current modulating a beam of electrons having a beam energy of kv. with the green record represented by the receivers green color information signal, both the first and second phosphors will be concurrently energized to produce a white or substantially achromatic light. Thus red and white images are produced on the viewing screen either continuously or alternately, by two electron beams moving in registry in a raster scanning pattern across the viewing screen. These images combine to form a a composite image which subjectively appears to include a full range of hues inhluding those which are not actually present in a colorimetric sense. Such a two-color system of presenting full color images is known in the art and provides images of pleasing appearance in which the hues appear more saturated than would be expected. Such a system is described in further detail in the copending and coassigned application Ser. No. 452,299, filed Apr. 30, 1965, now Pat. No. 3,371,153.

To obtain an even more desirable color display, a viewing screen is employed which also includes particles of a third phosphor having a higher beam energy threshold, e.g., one which emits a substantial level of light of a third color (e.g., blue) when energized by electrons having a higher velocity, e.g., 20 kv. As above, the third phosphor may begin to turn on at a lower voltage, perhaps at 15 kv., but much higher voltages are needed for an operating light level. A beam of such an energy level, modulated in accordance with the blue record represented by the blue color information signal of the television receiver, will energize all three phosphors and produce a third image of cooler achromatic light, and provide a composite image of particularly pleasing color. A more detailed description of such systems may be found in the copending and coassigned application Ser. No. 450,705, filed Apr. 26, 1965, now abandoned.

In copending application Ser. No. 459,582, filed May 28, 1965, now Pat. No. 3,408,223, the methods more particularly described individually coating the particles by physical deposition of a vapor phase material on the surfaces of the phosphor particles to provide an electron retarding barrier layer. In copending application Ser. No. 561,815, filed June 30, 1966, now Pat. No. 3,449,148, an improved method was disclosed for forming phosphors which are differently responsive to electrons of different energy levels or velocities, and thus are particularly useful in the above discussed color display systems. In accordance with the present invention, improved methods and apparatus are provided for forming phosphor particles having significantly superior characteristics, and viewing screens utilizing these phosphor particles are also disclosed.

SUMMARY OF THE INVENTION Among the several objects of this invention may be noted the provision of phosphors for use in making viewing screens for color display systems in which image colors are controlled by modulating or varying the energy level or velocity of an electron beam; the provision of simple, economical and reliable methods of making phosphors having improved characteristics; the provision of methods by which are formed on phosphor particles electron retarding surface barriers; the provision of methods of the class described in which phosphor particles of different color light-emitting properties may be provided with closely controlled electron energization thresholds; the provision of viewing screens for color display systems which include such improved phosphors; and the provision of improved apparatus for increasing the energization threshold of phosphor particles. Other objects and features will be in part apparent and in part pointed out hereinafter.

Briefly, this invention is directed to a method of increasing the electron energization threshold of phosphor particles, in which a thin substantially even layer of the particles is positioned in heat exchange relationship with a mass of a relatively high thermal conductivity and emissivity material having a surface supporting the particle layer. The area of the surface supporting the particles is at least equal to the area of the layer. Heat is supplied periodically to the particle layer and the material while exposing said particles to an atmosphere of a material which reacts in vapor phase with the phosphor particles to effect an in situ surface chemical reaction on the phosphor particle surfaces to form a surface barrier layer. These novel phosphor particles thus have a surface layer which constitutes a partial barrier to electrons. The light output versus electron energization level characteristic of said particles has a slope which is at least approximately 60% of the comparable slope of the characteristic of the particles without such a barrier layer and an initial energization tail which is not greater than about 40% of the magnitude of said barrier. Also encompassed by this invention is apparatus for increasing the electron energization threshold of phosphor particles which includes a reaction chamber having a mass of relatively high thermal conductivity and emissivity material within the chamber. A layer of phosphor particles is contained within means supported by said mass and in heat exchange relationship therewith. Means are provided for supplying to the interior of the reaction chamber an atmosphere of a material which reacts in vapor phase with the phosphor particles to effect an in situ surface chemical reaction on the phosphor particle surfaces to form a surface barrier layer. The apparatus also includes means for sensing the temperature of said particles and further means responsive to the sensing means supplying heat to said particle layer and said mass to effect cyclic variations in the temperature of the particles.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates apparatus of the present invention for forming phosphor particles with increased energization thresholds;

FIG. 2 is a transverse cross section of the apparatus of FIG. 1 showing in broken lines an open position of the apparatus;

FIG. 3 is a section taken on line 33 of FIG. 2;

FIG. 4 is a section taken on line 4-4 of FIG. 3, but on an enlarged scale;

FIG. 5 illustrates a time-temperature relationship utilized in carrying out a preferred embodiment of a method of this invention;

FIGS. 6-9 are graphical representations of the light output versus electron energization characteristics of phos phor particles prior and subsequent to formation of electron retarding surface barriers by methods of this invention;

FIG. 10 graphically illustrates improved characteristics of phosphor particles treated in accordance with the present invention relative to prior art phosphor characteristics;

FIG. 11 shows a portion of a viewing screen of a color display system employing phosphor particles of this invention; and

FIG. 12 illustrates additional apparatus of the present invention for forming phosphor particles with increased energization thresholds.

Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now more particularly to the drawings, a furnace is indicated generally at reference numeral 1. This furnace, preferably of the electrically heated type, includes an upper section 3 and a matching lower section 5 secured together at their rear edges by a hinge 7 and mounted on a base 9. A pair of actuating arms 11 interconnects furnace sections 3 and 5 to an internally threaded collar 13 which is driven by a threaded shaft 15 and a reversible drive motor 17 to open and to close the sections 3 and 5 in clam-shell fashion thereby to encompass a section of a quartz reactor tube 19. Tube 19 is supported by a pair of stands 21. Centrally positioned within the reaction tube is a slab 23 of a relatively high thermal conductivity and emissivity material. For example, this slab is formed of graphite having the surfaces thereof coated with a smooth uniform dense fused silicon carbide coating. The thermal conductivity of the slab is typically in the order of 0.2 cal.-cm./sec., cm. C. and its thermal emissivity is about 0.9. This mass or slab 23 supports a boat-shaped container 25 of quartz in which a thin substantially even layer 27 of phosphor particles is placed. Slab 23 is somewhat longer and approximately the same width as container 25 so that the surface area of the slab is at least equal to the area of the layer 27 of phosphor particles.

A thinner quartz tube 29 is aligned within the reactor tube, offset from the central axis and traversing the length thereof. It is supported within bores of end members 31. At least one end of tube 29 is connected to a source of wet oxygen 32 by flexible tubing 33, the other end of tube 29 being closed or commonly connected to source 32. Tube 29 has an electrically energized preheating sleeve 34, and a slit 35 through which wet oxygen is passed thereby exposing the particle layer 27 to an atmosphere substantially saturated with oxygen. These latter components constitute means for supplying to the interior of the reaction chamber or tube 19 an atmosphere of a material which will react in vapor phase with the phosphor particles and effect an in situ surface chemical reaction on the phosphor particle surfaces to form surface barrier layers. Excess oxygen is exhausted through and around end members 31.

The temperature of the particles in layer 27 is sensed by a thermocouple 37 connected by leads 38 to any customary type of controller 39 which energizes drive motor 17 via cable 40 when the temperature of layers 27 rises to a predetermined level and thereby opens the furnace sections 3 and 5 as shown in broken lines in FIG. 2, thus permitting the particles to cool until a lower predetermined temperature level is reached, whereupon controller 39 causes motor 17 to operate to close the furnace about the reaction chamber and reheat the phosphor particles.

As one specific example of a method of this invention wherein phosphor particles are formed which have an increased energization threshold, seven grams of a blue light-emitting phosphor, zinc sulfide (silver activated) such as that commercially available under the trade designation #1320 from Sylvania Electric Products, was placed in container 25 in an even thin layer. The size of these phosphor particles was about 15 to 20 microns and the layer thickness was about .050" or less. Oxygen bubbled through deionized water 42 was supplied via gas feed tube 29 and slit 35 to the interior of the reaction chamber at a rate of approximately 2 liters per minute. Controller 39 was adjusted to initially energize the furnace until the temperature of particles 27 rose to 560 C. and then operate drive motor 17 to open the furnace and permit the temperature of the particles to drop to 350 C., and repeat this procedure incrementally increasing the maximum temperature achieved in each successive cycle by 20 C. The graph of this timetemperature relationship is illustrated in FIG. 5, there being 4 cycles over a reaction time period of minutes. The particles thus treated were uniform and dark lemon yellow in color. The phosphor particles were thereafter exposed to an atmosphere of vapor phase sulfur for about 2 hours at a temperature of about 650 C., the sulfur being supplied by a hydrogen sulfide source or by heating elemental sulfur.

Both an untreated sample and a treated sample of these phosphor particles were comparatively tested by conventional procedures and apparatus, such as described in coassigned and copending application Ser. No. 459,582, filed May 28, 1965, now US. Pat. No. 3,408,223. The light output versus the electron energization level of the untreated phosphor particles is graphically shown in FIG. 6 by the determinative points and the straight broken line approximation thereof which has a slope of 1.19. The determinative points and the straight solid line approximation thereof represent the light output versus electron energization level characteristic of the thus-treated phosphor and this line has a slope of 0.94. The slope of the treated phosphor particles therefore is 79% that of the untreated phosphors. It is seen that the broken line representation of the intensity-electron energization level characteristic of the untreated phosphor particles intersects the axis of abscissas at 4'kv. and the solid line representation of this characteristic of the treated phosphor particles intersects this axis at 12.5 kv. Thus the phosphor particle surface layer formed in situ constitutes an electron barrier of 8.5 kv. as designated B. The initial energization tail represented by the series of points generally designated A illustrates that the treated phosphor particles began emitting blue light at about 9 kv. (point I). The energization tail T, which is the difference between the initial electron energization threshold I and the point of intersection F of the light outputelectron energization level characteristic for the treated phosphor particles is about 3.5 kv. Thus the energization tail is only about 41% of the magnitude of the barrier, 8.5 kv. in this instance. These results represent a marked improvement over prior art phosphor particles in which such barriers and slope and energization tail percentages could not be approached.

It will be noted that significant advantages result from positioning a thin even layer of phosphor particles on a mass of relatively high thermal conductivity and emissivity material, such as the susceptor or slab 23, and periodically supplying heat to the particles. These features overcome prior problems in increasing the electron energization threshold of phosphor particles, such as for example the tendency of the exothermic oxidizing reaction to result in overheating. and cause erratic and uneven results and produce phosphors with unsatisfactory characteristics. Due to the high conductivity and emissivity of the slab 23, the phosphor particles are uniformly heated and evenly and rapidly cooled thereby forming a very uniform oxidation layer on the surfaces of the particles. By periodically or cyclically heating the particles the in situ reaction proceeds without disadvantageous overheating. Moreover, it has been found in accordance with the present invention that the reaction temperature can be beneficially increased incrementally in subsequent. cycles of heating, thus effecting a reduction in overall reaction time for forming a given magnitude of barrier layer on the particles. It is believed that this increased temperature can be utilized as the reaction proceeds and without degrading thephosphor particles, because of the previous buildup of the oxidation layer which would appear to act as an insulating or protective layer. Thus, by increasing the maximum temperature incrementally each heating cycle, the oxidation rate can be maintained without degrading the phosphor.

As another specific example of the present invention, six grams of a blue light-emitting phosphor, zinc sulfide (silver activated) was placed in container in aneven thin layer and the particles were exposed to wet oxygen at the rate of 2 l.p.m. for two hours, the heat being applied cyclically for eight cycles with the minimum temperature being 350 C. and the maximum temperature per cycle being 550 C. The oxidized particles were of uniform dark lemon yellow color and were reacted for about two hours in an atmosphere of vapor phase sulfur at about 650 C. to reconvert at least partially the zinc oxide barrier layer to zinc sulfide. When tested by the same procedure described above these particles exhibited characteristics as shown in FIG. 7, That is, the barrier layer B was determined to be 10.1 kv.; the slope of the untreated and treated phosphor particles were 1.10 and 0.68 respectively (thereby providing a slope ratio of approximately 62%); and the initial energization tail T was 3.5 kv. or about of the barrier.

The results of another specific example of the present invention are represented by FIG. 8 wherein the previous example was repeated except that 4 grams of phosphor particles were used; only 7 rather than 8 temperature cycles were employed; and 3 rather than 2 liters per minute of wet oxygen were fed into the reaction furnace. The ratio of the slopes (0.55 for untreated and 0.47 for the treated particles) was approximately the barrier B was 9.3 kv.; and the initial energization tail T was 2.6 kv. or about 28% of the barrier.

Still another example is illustrated by FIG. 9 which represents the comparative characteristics of cyan or green light-emitting phosphor particles, zinc sulfide (48%) cadmium sulfide (52%) (silver activated) such as obtainable under the trade designation #1220 from Sylvania Electric Products, before and after the method of the present invention. In this instance, the previous example was repeated except that 6 grams of the phosphor particles were used; 3 l.p.m. of dry oxygen were employed; and the temperature of the particles was varied in 8 successive cycles in each of which the temperature range was 350 C. to 580 C. The time period was 45 minutes. The slope of the untreated phosphor was 3.91 and the slope of the treated phosphor was 65% thereof or 2.52. The barrier B was determined to be 5.7 kv. and the initial energization tail T was about 40% thereof, i.e., 2.3 kv.

Referring now to FIG. 10, the results of many different examples of the present invention are represented by curve C in which a fraction of the original efiiciency, or light output percentage, of the treated phosphor particles was plotted versus the energization barrier and compared with dashed line curve P which represents the same characteristics based on many examples of the best known prior art methods for increasing the electron energization threshold. It is to be noted that the efficiency of the phosphor particles treated in accordance with the present invention varies from well over 0.9 or of the light output of the untreated phosphor particles at relatively low (e.g., 2 kv.) barrier magnitudes to over 60% for relatively high barriers (e.g., 11 or 12 kv.) as compared to comparable light output efficiencies of phosphors treated in accordance with the best known prior art methods wherein the respective values varied from less than 0.80 or 80% to less than 0.1 or 10%.

Another apparatus suitable for performing the oxidizing process of the invention is the reciprocating furnace illustrated in a top sectional view in FIG. 12. In this apparatus, electrically heated furnace 50 includes heater coils 51 and a quartz tube 52 which defines a reaction chamber within the furnace. A silicon carbide coated graphite susceptor 53 is slideably positioned within the tube 52 and supports phosphor particles 55 on a flat surface thereof as shown. Quartz tube 56, connected to a source of wet oxygen (not shown), provides oxygen to the reaction chamber. Quartz tape or platinum wires 57 are attached to the susceptor 53, and the susceptor is moved through the reaction chamber by a reversible variable speed drive means (not shown). Photocells 58 and lights 59 are positioned at either end of the reaction chamber.

In operation, a temperature grade gradient is established within the chamber by the heater coils 51. Preferably, the temperature is between 550 C. and 650 C. in the middle of the chamber and about 350 C. at either end of the chamber. Thus temperature cycling of the phosphor particles is eflected by reciprocally moving the slab boat and phosphor particles through the furnace. The photocells and lights at either end of the chamber are operatively connected to the drive means to reverse the drive whenever the slab interrupts the light impinging on the photocells. The temperature gradient within the reaction chamber and the speed of the drive means can be adjusted to provide a time-temperature relationship such as shown in FIG. 5.

The phosphor particles of the present invention are particularly advantageous in making viewing screens for color television receivers or other types of color display systems. Such a viewing screen is illustrated in FIG. 11 in which cyan, or cyan and blue, light-emitting particles, each having a barrier layer to increase the electron energization threshold of the respective phosphors is randomly mixed with particles of a red light-emitting phosphor to form a thin closely packed layer 41 on a transparent glass face plate 43, which layer is swept by a narrow electron scanning beam 45. Each of the red and cyan (and blue if present) light-emitting phosphors has a different energy threshold so that, for example, the red light-emitting phosphor particles will be activated or excited at a relatively low electron beam energy level or velocity (e.g., kv. operating voltage) while the cyan light-emitting phosphor particles will begin to emit light at a somewhat higher voltage and be excited to give substantial amounts of cyan light when the beam electron velocity or beam energy level is increased substantially (e.g., to about 15 kv.). The blue light-emitting phosphor particles would require a still further increase in electron beam velocity to about kv. before a substantial amount of blue light is emitted. A more detailed description of such systems (including how the electron beam is currentmodulated at different beam energy levels by different records each corresponding to a different color image being instantaneously scanned) may be found in the copending and coassigned application Ser. No. 450,705, filed Apr. 26, 1965, now abandoned.

The in situ formed surface barrier layers on the phosphor particles treated in accordance with the present invention provide the different beam energy thresholds so that such viewing screens may be made for these color display systems. As the efficiencies of the treated phosphor particles of the present invention are high and the slopes are relatively steep and the initial energization tails are relatively short, superior viewing screens can be formed in accordance with this invention. Thus, phosphors having a barrier formed as described herein will have a light output which will increase relatively rapidly as the electron beam velocity increases above a lower level at which a different color phosphor will emit a substantial amount of light.

It will be understood that phosphor particles with compositions other than those defined in the above examples may be utilized and that the upper surface portion of susceptor or slab 23 may be recessed to contain a layer 27 of phosphor particles rather than using a separate container 25.

In view of the above, it will be seen that the several 8 objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above methods, constructions and products without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

We claim:

1. A method of increasing the electron energization threshold of zinc sulfide-containing phosphor particles comprising positioning a thin substantially even layer of said particles in heat exchanging relationship with silicon carbide having a surface area supporting said layer which area is at least equal to the area of said layer, and periodically supplying heat in the range of about 350 C. to about 600 C. to said particle layer and said silicon carbide while exposing said particles to an atmosphere sub stantially saturated with oxygen, whereby said oxygen re acts in vapor phase with the phosphor particles to effect an in situ surface chemical reaction on the phosphor particle surfaces to form a surface barrier layer comprising zinc oxide.

2. A method as set forth in claim 1 in which the heat supplied periodically to said phosphor particles is incrementally increased as said chemical reaction proceeds to elfect cyclic variations in the temperature of said particles whereby the temperature is varied from about 350 C. to 540 C. initially and the maximum temperature achieved in subsequent heating cycles is incrementally increased until a maximum temperature per cycle in the order of about 600 C. is achieved finally.

3. A method as set forth in claim 1 which includes the further step of heating the resulting phosphor particles to an elevated temperature and exposing them to a sulfur containing atmosphere, the sulfur in said atmosphere being supplied by a hydrogen sulfide source or by heating elemental sulfur, Which sulfur reacts in vapor phase with the thereby formed barrier layer to eifect an in situ reaction therewith and form a sulfide-containing barrier layer.

References Cited UNITED STATES PATENTS 2,908,588 10/1959 Harper 252301.6X 3,113,929 12/19 63 Koury 2523()l.6 3,249,552 5/1966 Lehmann 252301.6

ALFRED L. LEAVITT, Primary Examiner W. F. CYRON, Assistant Examiner US Cl. X.R. 117-335 

